Adding ADF's or snails is all about your bettas personality. Some don't give a care in the world about them, some attack them shreds. If you would liek something more entertaining, then the ADF's. If not, then a snail.
Snails are pretty entertaining. We got one for the betta tank but had to rehome him to a different tank since the betta kept him pinned in the corner. We end up watching snail-vision now.
You could also try some types of shrimp, though Mr. Betta might go after them as well.
I'm not too partial to shrimp they're too bug-esque for my tastes. lol. When he was in my ten gallon he was fine with the corydoras catfish... does that mean he'll be fine with the frog?